A fuel cell is a kind of power generating device generating electric power by oxidizing fuel such as hydrogen and methanol electrochemically. In recent years, attention is paid to the fuel cell as a source for supplying clean energy. Fuel cells are classified by the type of electrolyte, into phosphoric acid type, molten carbonate type, solid oxide type and solid polymer electrolyte type. Among them, the solid polymer electrolyte fuel cell (also referred to simply as “PEFC”) is a fuel cell arranged to generate power by supplying hydrogen to one side and oxygen to the other side of a membrane electrode assembly (also referred to as “MEA”) including electrodes on both sides of an electrolyte membrane. Since PEFC can provide an output density comparable to an internal combustion engine, research is widely performed for practical use as a power source for an electric vehicle and other applications.
In general, PEFC is in the form of fuel cell stack including a plurality of unit cells each including integrally a solid polymer electrolyte membrane, and hydrogen side and oxygen side electrodes confronting each other across the solid polymer electrolyte membrane. These unit cells are stacked through separator(s). Between each separator and an adjacent electrode, there is provided a gas diffusion layer of porous material normally having an electric conductivity. The gas diffusion layer is arranged to undertake a role to enable stable exchange of hydrogen, oxygen, water, electrons, heat etc., among the electrode layer and an external circuit.
As a fuel cell for vehicles, wide use is made of a stack type fuel cell including a stack of unit cells each of which includes a sheet-shaped MEA and a sheet-shaped separator. Normally, the thickness of a unit cell is smaller than or equal to 10 mm. Within this thickness, the unit cell is required to allow simultaneous flows of various fluids including a fuel gas and an oxidizing gas, and further including other fluid (such as a cooling water) in some cases. Therefore, the unit cell requires a complicated seal structure provided for each fluid passage, and this requirement contributes to deterioration of the productivity of fuel cells.
As such seal technique, there are known a technique using a repulsion force of an elastic member, a technique using adhesion or sticking, a technique using fixing or pressing with compressive material, and a technique using mechanical deformation such as staking or caulking. Among these, the technique using the repulsion force of the elastic material is widely used because of advantages, 1) high reliability, 2) high durability, and 3) possibility of exfoliation or peeling. However, this technique is limited in the reduction of the thickness and the size of the fuel cell because of the necessity of a predetermined margin for contraction or squash.
The technique using, as sealing material, the adhesive (liquid material after coating; and adhesiveness is achieved by hardening or curing) is advantageous for the reduction of thickness and size since the margin for contraction or squash is not needed. However, there is a need for preventing contact of material other than the material to be bonded, to the surface coated with the adhesive before hardening. Moreover, at the time of layer stack of unit cells coated with the adhesive, the adhesive is flowable until hardening. Therefore, though minute position adjustment is possible, the lamination is liable to shift to a deviated position by external cause or other influence.
The technique using, as sealing material, the sticky agent (gel-like solid material after coating; and adhesiveness is achieved by pressure; also called pressure sensitive adhesive) is advantageous for the reduction of thickness and size since the margin for contraction or squash is not needed. United States patent application publication 2002/068211A discloses a technique using the sticky agent.